In the subject type of spectrophotometers for liquid chromatography (LC) a substance whose quantitative presence in a sample is to be determined is dissolved in a suitable carrier solvent, separated in an LC column and flowed through a detector cell which has end windows through which ultraviolet or visible light radiation is directed. Radiation exiting from the cell falls on a photodetector whose output is recorded by suitable instrumentation which is calibrated to indicate the amount of radiation absorbed by the fluid flowing through the cell. Absorbance is customarily indicated by a graph continuously recorded on a strip chart by a pen recorder. The quantitative presence of a substance of interest is determined by measuring the area under the graph peaks which represent the amount of radiation of a particular wavelength that is absorbed, particular materials being identified by particular wavelengths characteristically absorbed by them.
The sensitivity of a spectrophotometer detector cell is a function particularly of the stability of the base line of the graph; the graph base line is established by the absorbance of the solvent used, and will change in relation to any change in the refractive index of the solvent, which in turn will be changed by a change in the temperature of the solvent in the cell. When the baseline changes, the true peak area can not be measured accurately and the peaks themselves become less clearly defined and hence difficult to identify and measure with any reliable degree of accuracy. The sensitivity of the instrument which is rather limited in the best circumstance when the index of refraction of the solvent remains constant (i.e. when flow noise is at a minimum) is disproportionally reduced by any change in temperatures of the solvent, thus altering its index of refraction and increasing flow noise.
U.S. Pat. No. 4,192,614 discloses a spectrophotometer cell assembly including a cell defined by a bore through a body and closed at the ends by radiation transparent windows with inlet and outlet passages through the body to the bore, so that radiation passed through the sample fluid flowing through the bore is detected by a photodetector. The body, which is expressly disclosed to be made of a thermally conductive material, is a large thermal mass in relation to the volume of the cell. A tubular inlet conduit, also made of a thermally conductive material, wraps around the body and connects to the inlet passage so that fluid flowing into the bore will tend to equilibrate and reach a stable temperature due to the heat sink effect of the body and conduit, thereby stabilizing the refractive index of the fluid in the bore and enhancing the sensitivity of the photodetection.
U.S. Pat. Nos. 4,598,765 and 4,589,477 also involve such apparatus and are similarly concerned about equilibrating temperature. Each of the three aforementioned patents teaches a complex means for heat exchanging with respect to the inlet conduit and a thermally conducting housing that forms the cell. Because of such construction with corrosion resistant metal cells with relatively small size and a requirement for precision, the prior art apparatus is quite difficult and expensive to make. Problems with the equilibration construction also include fluid mixing in the inlet tube length causing increased bandwidth of a fluid sample, and incomplete equilibration for high fluid flow rates. Also, a long inlet tube associated with the heat exchanger is more susceptible to blockage.
Reliability problems have been encountered in sealing the cells. A transparent window such as an optical lens must be sealed into the metal cell. This has been effected by a gasket such as described in U.S. Pat. No. 4,121,859. A specially angled surface in the cell body is also required, further adding to cost. The gasket has been prone to distorting or breaking thus causing blockage of fluid flow and optical path.
Prior art LC detectors, as represented by the aforementioned patents (which are assigned to the same assignee as the present invention), have become quite accurate. However, a significant requirement for further improvement exists.